1. Field of the Invention
The present invention relates to a lead frame for semiconductor devices, and more particularly, to a lead frame in which a protection layer for protecting a nickel thin film layer deposited on the upper surface of a metal substrate is improved in applying a preplating method.
2. Description of the Related Art
Like semiconductor chips, a lead frame is essential to a semiconductor package, and supports a semiconductor chip and electrically connects the chip to an external circuit.
FIG. 1 shows an example of such a lead frame. As shown in FIG. 1, a lead frame 10 is comprised of a pad 11, inner leads 12, and outer leads 13. This lead frame can be usually manufactured by stamping or etching.
As shown in FIG. 2, a semiconductor chip 40 is installed on the pad 11, and wire-bonded to the inner leads 12. The outer leads 13 are electrically connected to an external circuit. Then, the chip 40 and the inner leads 12 are molded by resin 14 to complete a semiconductor package 15.
During the manufacture of the semiconductor package, the pad 11 and the ends of the inner leads 12 to be wire-bonded to the chip 40 are plated with a metal such as silver in order to provide a good wire bond property between the chip 40 and the inner leads 12 and good characteristics of the pad 11. Also, a predetermined area of the outer leads 13 is soldered, i.e., plated with tin-lead (Sn--Pb) to improve solderability for mounting the semiconductor package on a substrate. However, such a process requires a wet process after the resin molding process, which degrades the reliability of completed products.
In order to solve the above problem, a preplating method has been proposed for pre-coating a solder-wettable material on a lead frame and forming an intermediate plated layer before a semiconductor packaging process.
FIGS. 3 and 4 show examples of a conventional lead frame manufactured by the preplating method. As shown in FIG. 3, a nickel thin intermediate layer 22 and an outermost palladium thin layer 23 are sequentially stacked on a metal substrate 21 made of copper or a copper alloy. As shown in FIG. 4, a thin layer 24 made of gold is formed on the upper surface of the palladium thin layer 23.
In a lead frame 20 of FIG. 4 configured as described above, the nickel thin layer 22 prevents copper or iron in the metal substrate 21 from diffusing to the surface of the lead frame, to thereby form copper oxides or copper sulfides. The palladium thin layer 23 prevents oxidation of the surface of the nickel thin layer 22, and the gold thin layer 24 formed on the upper surface of the palladium thin layer 23 improves solderability.
However, during the manufacture of the lead frame 20, when there is damage to the surface of the metal substrate 21, a nickel layer on the damaged portion is more rapidly plated than on other neighboring portions due to a high surface energy of the damaged portion, which degrades cohesiveness of the damaged portion to the neighboring portion. In particular, in case that the surface of the nickel thin layer formed on the damaged portion is electrically plated with the palladium thin layer, a large amount of hydrogen is entrained during deposition of palladium because of the similarity between a palladium precipitation potential and a hydrogen precipitation potential, whereby damage to the palladium thin layer is accelerated. The damage to the palladium thin layer causes oxidation of the nickel layer to thus degrade solderability. Also, solderability can be degraded due to diffusion of components between plated layers caused by heat applied during a semiconductor manufacturing process.
A lead frame provided to solve the above problem has been disclosed in U.S. Pat. No. 5,360,991, in which a nickel thin layer is formed on the upper surface of a base metal, and a composite protection layer is formed on the upper surface of the nickel thin layer. The composite protection layer is comprised of a palladium or soft gold strike layer, a palladium-nickel alloy layer, a palladium layer, and a gold layer which are sequentially stacked on the nickel thin layer.
An example of another lead frame has been disclosed in U.S. Pat. No. 5,436,082, in which a nickel layer, a copper layer, a silver layer, and a palladium layer are sequentially stacked on the upper surface of a base metal.
The protection layer of each of the above-described conventional lead frames is comprised of a plurality of layers, resulting in a complicated manufacturing process and involving thermal migration from the protection layer to the outermost layer.
In an example of still another conventional lead frame, a gold thin layer can be formed on part of the external leads to protect the outermost palladium layer. In this case, the gold thin layer protects the palladium thin layer and also can reduce a wetting time by expediting smooth dissolution of palladium and lead upon initial wetting. However, use of a gold thin layer increases production costs and provides bad adhesiveness to molding resin, thus degrading the reliability of the semiconductor package. Also, there is a limit in making up for large and small cracks formed on a metal substrate during banding of the lead frame, and these cracks accelerate corrosion and oxidation of the lead frame. Furthermore, the gold thin layer forms Au--Sn together with tin (Sn) of a solder, thereby degrading solderability for mounting a semiconductor package on a substrate.